The present invention relates to a process for the recovery of oil by emulsion flooding wherein an anionic tenside is employed as the emulsifier.
In the extraction of oil from oil-bearing deposits, generally, it is possible only to recover a fraction of the originally present oil by means of primary extraction methods. In such procedures, the oil reaches the surface due to the natural reservoir pressure. After exhaustion of this primary energy, the oil yield can be increased by secondary measures. In this method, water is forced into one or several injection wells of the deposit, and the oil is pushed to one or several production wells and thereafter brought to the surface. This so-called water flooding as a secondary measure is relatively inexpensive and accordingly is frequently employed, but leads in many cases only to a minor increase in oil extraction from the deposit.
An effective displacement of the oil, which is more expensive but urgently required from an economic viewpoint because of the present scarcity of petroleum, can be accomplished by tertiary measures. These are understood to mean processes wherein either the viscosity of the oil is reduced and/or the viscosity of the flooding water is increased and/or the interfacial tension between water and oil is lowered.
Most of these processes can be classified either as solution or mixture flooding, thermal oil recovery methods, tenside or polymer flooding and/or as a combination of several of the aforementioned methods. Thermal recovery methods include the injection of steam or hot water and/or are accomplished as in situ combustion. Solution or mixture processes involve injecting a solvent for the petroleum into the deposit. The solvent can be a gas and/or a liquid.
Tenside flooding processes are based primarily on a strong lowering of the interfacial surface tension between oil and flooding water. These can be categorized in dependence on tenside concentration and in some cases on type of tenside and additives, among tenside-supported water flooding, customary tenside flooding (low-tension flooding), micellar flooding, and emulsion flooding. In some instances, especially in the presence of relatively high tenside concentrations, water-in-oil emulsions are produced having a markedly increased viscosity as compared with the oil. Thus, a purpose of the tenside flooding step also is a reduction in the mobility relationship, i.e., an increase in oil mobility. This is especially advantageous in case of oils whose viscosity is not very low. Thereby fingering and passage of the driven water is reduced and, thus, the degree of efficiency of oil displacement is raised.
Pure polymer flooding is based predominantly on the last-described effect of a more favorable mobility ratio between oil and the pursuing flooding water.
The mode of operation of emulsion flooding, on the one hand, is aimed at oil mobilization by lowering interfacial tension. The limiting case can be deemed to be a phase miscibility within the framework of microemulsion flooding. On the other hand, emulsion flooding is also based on improving the volumetric flooding effect by rendering the carrier permeability uniform. For this purpose, the more or less viscous emulsion penetrates preferably into the more permeable carrier strata and thus permits an improved coverage of the low-permeability zones by the following flooding medium. In the literature, this second effect is emphasized in most instances. Thus, U.S. Pat. No. 3,527,301 and German Pat. No. 1,234,646 describe the use of fatty alcohol or alkyl phenol ethoxylates as emulsifiers which permit the attainment of a marked increase in the degree of oil extraction.
These compounds, as is known, have the advantage of being compatible with salts, even with higher-valence metal cations. Fundamental laboratory tests on high-permeability artificial formations have shown, however, that when using these nonionic tensides in emulsion flooding, extremely high pressure gradients occur which render field utilization impractical (B. Hofling, "Erdoel-Erdgas-Zeitschr." [Petroleum-Natural Gas-News] 81:480 [1965]).
In many more recent processes, disclosed, for example, in DOS [German Unexamined Laid-Open Application] No. 2,456,860 or U.S. Pat. Nos. 4,192,382 and 4,194,563, complicated tenside mixtures are used as emulsifiers which are compatible even with waters of relatively high salinity if the temperatures are not too high. In part, the emulsions are formed in situ, the organic phase and the aqueous emulsifier solution being injected in succession into the formation. Tenside mixtures enabling the formation of optimum emulsions generally do not lower the interfacial tension between the oil phase and the water phase to a sufficient degree and therefore are unsuitable for mobilization of the residual oil (U.S. Pat. No. 4,184,549). Consequently, in the above-mentioned conventional methods still other tensides or tenside mixtures are required for emulsion flooding in addition to the tenside mixtures which are effective as the emulsifiers. The other tensides or tenside mixtures effect oil-mobilization due to their high surfactant activity. Considering the fact that varying tenside molecules interact in various ways with the rock surface, it can certainly be assumed that the composition of the tenside mixture during transport through the formation will constantly change. As a result, there is a loss of the careful adaptation of the tenside combination to the properties of the reservoir.
An optimum process for emulsion flooding, therefore, has as a prerequisite, a maximally uniform tenside as emulsifier, which enables the formation of stable emulsions and, furthermore, adequately lowers the interfacial tension of the water/reservoir oil interface and thus is oil-mobilizing.
Furthermore, the tenside must be compatible with water of relatively high salinity; especially the presence of alkaline earth metal ions must not exert a very negative influence. The tenside must be thermally stable over long periods of time and is to be absorbed on the rock surface, under reservoir conditions, only to a minor extent. Of course, no pressure gradients uncontrollable under practical conditions must occur during emulsion flooding.